Ruthenium Electrodeposition from Non-Aqueous Electrolytes Containing Divalent Ions

Abstract

Ruthenium as a metal is characterized by interesting technological and aesthetic properties, which make it an ideal choice for many applications. Ruthenium is dark grey in appearance, it presents a reduced tendency to corrode, low contact resistance and high hardness. When applied in the form of thin films, it can be used in jewels production, protection of electrical contacts, deposition of barrier layers in microelectronics and manufacturing of interconnects in integrated circuits [1].Ruthenium layers can be advantageously deposited exploiting electrodeposition, typically from aqueous sulfamate based electrolytes containing the metal in the form of a complex known as μ-nitridobisaquatetrachlororuthenate [Ru2(μ-N)(H2O)2Cl8]3-. These baths are routinely used at the industrial level and provide good coatings at acceptable cathodic efficiencies. However, ruthenium tends to oxidize in aqueous environment and the resulting deposits can contain impurities or oxides. In addition, the electrodeposition of ruthenium in water-based electrolytes presents difficulties when thick and crack-free coatings are required.In an attempt to solve these limitations, the use of non-aqueous electrolytes for the deposition of ruthenium coatings has been proposed. The substitution of water with alternative solvents can potentially relieve the problem of ruthenium oxidation. Moreover, non-aqueous electrolytes are characterized by wide potential windows, which translate into a reduction in the entity of parasitic hydrogen evolution reactions. Consequently, the properties of the deposit obtained can be potentially improved. In the context of non-aqueous electrolytes, ruthenium has been deposited from water and air stable ionic liquids [2, 3] and from deep eutectic solvents [4].With respect to our previous paper on the deposition of ruthenium [4], where the metal was deposited from the (IV) oxidation state, the present work describes a diametrically different approach. Indeed, we investigate the electrodeposition of ruthenium layers from a non-aqueous electrolyte based on ethylene glycol and containing the metal in its divalent state. The (II) oxidation state is obtained by reducing trivalent RuCl3 with ascorbic acid. A complete physical and electrochemical characterization of the resulting electrolytes is performed, demonstrating the reduction of Ru(III) to Ru(II). The deposition of thin and thick layers is attempted and the resulting coatings are characterized to exclude the possible presence of oxides or other secondary phases. Finally, the possible application of ruthenium layers for electrical contacts protection is evaluated.[1] R. Bernasconi and L. Magagnin, J. Electrochem. Soc. 166(1), D3219-D3225 (2019)[2] O. Raz, G. Cohn, W. Freyland, O. Mann and Y. Ein-Eli, Electrochim. Acta 54, 6042 (2009)[3] O. Mann, W. Freyland, O. Raz and Y. Ein-Eli, Chem. Phys. Lett. 460, 178 (2008)[4] R. Bernasconi, A. Lucotti, L. Nobili and L. Magagnin, J. Electrochem. Soc. 165(13), D620-D627 (2018)